Magnetic thin-film media, wherein a fine grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetic domains of the grains of the magnetic material. FIG. 1 shows a disk recording medium and a cross section of a disc showing the difference between longitudinal and perpendicular recording.
Perpendicular recording media are being developed for higher density recording as compared to longitudinal media. The thin-film perpendicular magnetic recording medium comprises a substrate and a magnetic layer having perpendicular magnetic anisotropy, wherein the magnetic layer comprises an easy axis oriented substantially in a direction perpendicular to the plane of the magnetic layer. Typically, the thin-film perpendicular magnetic recording medium comprises a rigid NiP-plated Al alloy substrate, or alternatively a glass or glass-ceramic substrate, and successively sputtered layers. The sputtered layers can include one or more underlayers, one or more magnetic layers, and a protective overcoat. The protective overcoat is typically a carbon overcoat which protects the magnetic layer from corrosion and oxidation and also reduces frictional forces between the disc and a read/write head. In addition, a thin layer of lubricant may be applied to the surface of the protective overcoat to enhance the tribological performance of the head-disc interface by reducing friction and wear of the protective overcoat.
Granular perpendicular recording media are being developed for its capability of further extending the areal recording density as compared to conventional perpendicular recording media which is limited by the existence of strong exchange coupling between magnetic grains. In contrast to conventional perpendicular media wherein the magnetic layer is typically sputtered in the presence of inert gas, most commonly argon (Ar), deposition of a granular perpendicular magnetic layer utilizes a reactive sputtering technique wherein oxygen (O2) is introduced, for example, in a gas mixture of Ar and O2. Not wishing to be bound by theory, it is believed that the introduction of O2 provides a source of oxygen that migrates into the grain boundaries forming oxides within the grain boundaries, and thereby providing a granular perpendicular structure having a reduced exchange coupling between grains. However, the oxide formation and distribution in the magnetic layer has been found to be insufficiently uniform, so that magnetic clusters form with varying intergranular exchange strength, causing degradation in the recording process.
In fabricating high signal-to-noise ratio (SNR) magnetic recording media, it is desirable that the magnetic particles or grains of the magnetic layer(s) be of uniformly small size, with a small, uniform amount of exchange coupling between the magnetic particles or grains. The optimal value of the exchange coupling is different for longitudinal and perpendicular recording media, e.g., a higher exchange coupling is desired for perpendicular media. However, in each instance a constant small value of exchange coupling between neighboring magnetic particles or grains is desired.
A low value (i.e., small amount) of exchange coupling between neighboring magnetic particles or grains is desired in order that magnetic switching of the magnetic particles or grains does not become too highly correlated. Reducing the exchange coupling decreases the sizes of the magnetic particles or grains, i.e., the sizes of the magnetic switching units. The cross-track correlation length and media noise are correspondingly reduced. However, near-zero exchange coupling between magnetic particles or grains produces a very low squareness-sheared M-H hysteresis loop, a broad switching field distribution, decreased resistance to self-demagnetization and thermal decay, and low nucleation fields (Hn) in perpendicular media designs. Non-uniform exchange coupling allows some magnetic particles or grains to act independently, with small particle or grain size, while other magnetic particles or grains act in clusters, resulting in broad distributions of particle or grain size and anisotropy field.
A method of reducing exchange coupling in a uniform magnetic layer involves sputtering from a target already having stable oxide material incorporated. Therefore, from the beginning of the magnetic layer growth, well-separated grains are formed without clustering effects, and intergranular exchange is reduced. However, complete elimination of exchange coupling would result in writability issues due to a widened switching field distribution.
Accordingly, there exists a need to control the degree of intergranular exchange coupling in perpendicular magnetic recording media in order to optimize writing capability without invoking excessive media noise. There exists a particular need to control the degree of intergranular exchange coupling in a uniform manner.